1. Field of the Invention
This invention relates to ceramics, and more particularly to improving the properties of ceramic zirconia.
2. Description of the Previously Published Art
Because of its toughness, wear resistance, hardness, low thermal conductivity, and other properties, zirconia (ZrO.sub.2) has found numerous ceramic applications. Typical of these uses (e.g., in gasoline or diesel engines) are wear buttons for valve tappets; valve seats; oxygen sensor sleeves; piston caps (for diesels), and precombustion chamber elements (for diesels). Typical non-auto engine uses include grinding balls, dies, check valves and the like.
Zirconia competes with or has replaced other ceramics for the above uses.
In all of the above uses the zirconia is prepared in a particular crystal morphology, viz., in tetragonal form. There are three commonly occurring and established crystal forms of zirconia: cubic, tetragonal, and monoclinic. Cubic is the normal form at high temperature. The tetragonal form can exist at room temperature, but is metastable and under stress tends to transform to the monoclinic form, with increase in volume and loss of various important properties. The form of zirconia desired for practically all the above mentioned ceramic end uses is the tetragonal. However the tetragonal in the unstabilized form, unfortunately, tends to convert to the less desirable monoclinic form, even on standing at room temperature. This conversion is also grain size dependent, the larger the grain size the easier the transformation. Transformation may be quite rapid under stress unless the zirconia is pre-stabilized in some way. It is of course also temperature dependent.
Various modifications and/or treatments of zirconia have been tried in efforts to minimize conversion of tetragonal to the monoclinic form. One approach is to use extremely fine zirconia powder such as less than 200 Angstrom units as reported in Jour. Phys. Chem, 69, 1238 (1965). Another approach is to add one or more stabilizers. The addition of yttria (Y.sub.2 O.sub.3) and ceria (CeO.sub.2) to stabilize the tetragonal zirconia system has been reported in Jour. Mat. Sci., 20, 3988-3992 (1985). In U.S. Pat. No. 4,753,902 zirconia is stabilized with two components. The first is from 5 to 45 mole percent titania. The second can be either (a) up to 10 mole percent of rare earth oxide, (b) up to 7 mole percent yttria or (c) up to 20 mole percent ceria.
3. Objects of the Invention
It is an object of the invention to provide a stabilized tetragonal zirconia which is cost effective. These properties include improved flexural strength, low temperature stability, fracture toughness, and hardness; improved resistance to thermal shock, abrasion and erosion; and others.
It is a further object to stabilize ceramic zirconia with dysprosia, ceria, and a third component which is either yttria or titania.
It is also an object to provide shaped zirconia ceramics of superior thermal and mechanical properties.
These and further objects will become apparent as the description of the invention proceeds.